Ductile concretes called “ultrahigh-performance” concretes are used especially for the construction of prestressed or non-prestressed concrete elements requiring superior mechanical properties, especially a high compressive strength. These concretes have a high flexural strength, typically at least 20 MPa, and a 28-day compressive strength of at least 120 MPa and a 28-day elastic modulus of greater than 45 GPa, these values being given for a concrete stored and maintained at 20° C.
To improve the mechanical characteristics of these concretes, various solutions have been recommended.
Thus, WO 95/01316 proposes the incorporation of metal fibers in a controlled amount and having dimensions chosen within defined proportions with respect to that of the aggregate particles constituting the matrix of the concrete.
The subject matter of WO 99/28267 also relates to ultrahigh-performance concretes containing metal fibers. To improve the mechanical strength of the concretes, especially their behavior both with respect to the initiation of microcracks and to the propagation of macrocracks, that document proposes the incorporation into the cementitious matrix of particles improving the toughness, these being chosen from acicular or flakelike particles having a mean size of at most 1 mm.
The acicular particles mentioned are mineral fibers, such as wollastonite, bauxite, mullite, potassium titanate, silicon carbide, calcium carbonate and hydroxyapatite fibers, or organic fibers derived from cellulose, these fibers optionally being able to have a surface coating of a polymeric organic compound.
The subject matter of WO 99/58468 relates to ultrahigh-performance concretes containing organic fibers such as reinforcing fibers so as to improve the ductility of these concretes. In that application, ultrahigh-performance concretes are also envisaged in which some of the organic fibers are replaced with metal fibers. It is also described that the organic fibers modify the fire behavior of the concrete.
The very-high-performance concretes described above, because of their mechanical properties, possess, however, insufficient fire resistance, this being manifested at best by spalling of the structures exposed to fire and possibly even explosion of these structures due to the vapor pressure of the water, which is physically or chemically bound by the constituents of the matrix, owing to the effect of the heat.
U.S. Pat. No. 5,749,961 proposes to improve the fire-resistance properties of compositions for high-performance fiber-free concretes having compressive strengths of around 90 to 105 MPa by the addition, into these compositions, of a combination of precipitated silica and of fibers capable of forming, by dissolution, softening, decomposition, shrinkage or melting, a network of capillary pores having a diameter of at least 10 μm and a length of at least 5 mm. However, one of the means mentioned in that patent and widely practiced in refractory concretes, which consists in introducing organic fibers into the concrete, firstly has the effect of seriously decreasing the mechanical strength of the hardened concrete, since the fibers introduce a smaller volume of elasticity than that of the matrix. Secondly, the rheological properties of the concrete in the fresh state are seriously reduced by the presence of the organic fibers in the composition, and are characterized by a low spread.
It therefore becomes difficult to conceive of applying such solutions to ultrahigh-performance ductile concretes as described in patent applications WO 99/28267 and WO 99/58468, which already recommend fiber volumes of around 2%.
It is important to be able to have compositions for ultrahigh-performance concretes having a rheology range which can go from plastic behavior to fluid behavior. Such concretes conventionally have a spread value of at least 150 mm, the spread value being measured by the shock table technique, a standardized technique used in general for mortars.
Nevertheless, until now, such concrete compositions have the drawback of exhibiting mediocre fire resistance.
Until now, attempts to improve the mechanical properties of ultrahigh-performance concretes have had deleterious effects on the fire resistance. Conversely, the solutions proposed for improving the fire behavior of concretes have the effect in general of decreasing the mechanical and/or rheological properties of these concretes in the unhardened state.
There is therefore no satisfactory solution to the problem of fire resistance of ultrahigh-performance concretes containing fibers, compatible with the desired properties of these concretes, namely high tensile/flexural strength, high compressive strength and rheology of the concrete in the unhardened state able to range from plastic behavior to fluid behavior.